The present invention is directed to medical instrument, particularly to a dental instrument having a hollow shaft which is rotatably seated in a head housing for the acceptance of a chucking means of a fixture for an internally-cooled tool which has a longitudinal channel for delivery of a coolant to an end of the tool, the instrument having a delivery channel for delivering coolant to an admission opening of the longitudinal channel at an end face of the shaft in a non-contacting fashion.
European Patent Application 0 455 452-A1 discloses a dental handpiece having a chucking means for receiving a tool having a hollow longitudinal channel. The handpiece also has a delivery channel for delivering cooling water to an admission opening of the tool in a free jet without contact therewith. Moreover, a part of the drive air which is conducted out of the instrument via an exhaust channel as exhaust or return air after flowing through the turbine drive is conducted through a ball bearing arrangement into a space wherein a pump member rotating together with the hollow shaft is located. Worm-shaped baffles are located in the inside of the housing for the pump member, and these baffles pump the extracted exhaust air into the channel of the tool. Used exhaust air, which usually also has dirt particles and lubricants included in it, is conducted into the longitudinal channel of the tool by this pump means. Apart from the fact that this is extremely undesirable for hygienic reasons, such an arrangement also has other disadvantages. The pump action, thus, only occurs when the exhaust air is fully available, for example when the turbine is rotating at an adequate speed. Given standstill of the tool or given a plugged exit opening at the tool as well, it can easily occur that the supplied coolant, usually water, will be pressed back through the pump arrangement and, thus, into the bearings.
A further disadvantage of such an embodiment may be seen wherein the arrangement is suitable for only one rotational sense of the tool. Given a change in rotational direction or sense, an undesirable suction effect could occur instead of the pump effect.
U.S. Pat. No. 4,021,920, whose disclosure is incorporated herein by reference thereto and which claims priority from German Application 23 31 023, discloses an arrangement wherein the delivery tube for the coolant is immersed into the longitudinal channel of the tool when the tool is put in place. A sealing ring is arranged in the tool to provide sealing of the cooling water from the remaining parts of the head housing. Such an embodiment has the disadvantage that the seal produces relatively high friction between the tool and the stationary delivery tube and will result in the useful life of the tool being shortened. Moreover, such a coolant delivery is only suitable for extremely low speeds. Another disadvantage may be seen wherein the instrument can only be employed for internally-cooled drills and that drills and instruments must be structurally matched to one another.
U.S. Pat. No. 5,096,421, whose disclosure is incorporated herein by reference thereto, discloses another design wherein the delivery of the coolant occurs in the region of the pressure cover of the treatment head. A membrane, which is provided with a bore, is clamped in a chamber of the head housing between the exit opening of the delivery channel and the admission opening in the tool shaft. This membrane presses in a sealing fashion against the end face of the tool shaft during operation, wherein the tool rotates, and this tends to create a seal with respect to the remaining housing parts, particularly the bearings for the hollow shaft.
The disadvantage of this arrangement is also present because a relatively high wear is established as a consequence of the friction between the drill shaft and membrane. In addition, an adequately reliable separation of the cooling path from the neighboring ball bearings is not established. Thus, the coolant can proceed into the bearings or, respectively, oil can proceed from the bearings into the longitudinal channels of the tool.